The present invention relates to the preparation of electrically insulative separators for disposing between the electrodes of an electrochemical cell. It is particularly applicable to the new generation of electrochemical cells that operate at high temperatures with molten salt electrolytes and other highly corrosive and reactive materials. For instance these separators are applicable for use in the alkali-metal, metal-sulfide cells.
Both porous rigid sheets and porous flexible fabrics of high-temperature and chemically inert ceramic materials have been considered for separator use. Ceramics such as BN, Y.sub.2 O.sub.3, MgO, MgAl.sub.2 O.sub.4, CaZrO.sub.3, Li.sub.2 ZrO.sub.3, LiAlO.sub.2, Si.sub.3 N.sub.4 and AlN have been suggested for use in the corrosive high-temperature environment of the high-energy electrochemical cells. However many of these materials are difficult to fabricate into cloths, felts or other fabrics and their use as a separator material has been limited to the porous rigid sheets or merely to layers of ceramic powder. Materials such as boron nitride and yttria can be made into flexible fabrics, felts or cloths but these materials and the processes for their fabrication contribute significantly to the cost of a cell.
The porous rigid sheets can be prepared from most of the ceramic materials including those such as magnesium oxide which are moderately priced. Straight forward procedures involving pressing and sintering steps are used. However, separators formed by these techniques have been found to be frangible, of insufficient porosity and often warped with inconsistent thickness. These problems along with the requirements for chemical and mechanical stability in the corrosive environment of the high-temperature, molten salt batteries have made it difficult to prepare a suitable separator in a porous, rigid ceramic form.